Have you ever looked at a design you created and wondered why over time it was looking tired and worn? Part of your answer may be the Mohs scale of mineral hardness.
What is the Mohs Harness Scale?
The Mohs Hardness scale is used by gemologists and geologists to rate minerals from 1 to 10 in order to describe the stone’s hardness and scratch resistance - it's based on the scratch test. For example, in the image below it shows that a fingernail has a Mohs Hardness of 2.5, thus with your fingernail you can scratch lower harness minerals such as talc and gypsum, but not the those above 2.5 like fluorite or quartz.
When is a Stone Considered to be Soft or Hard?
The Mohs sweet spot is 7.0 which is the hardness of Quartz, anything lower is considered a ‘soft’ stone and is more likely to scratch from normal use. Anything 7.0 or higher are considered to be hard stones. For those stone beads with a lower Mohs score, you may find it more likely that the stone has been stabilized in order to cut the bead and so that it stands up to normal wear and tear.
Hard Stones vs Soft Stones
Precious gems like Diamond, Topaz, Ruby, and Sapphire are all at the high end of the Mohs scale (8-10). It is one of the reasons why we see these stones more often in engagement rings. They are long lasting and are well suited to be worn on a daily basis with the least likelihood of scratches or chips over time.
On the other end of the scale are stones like calcite, selenite, and fluorite and require special care during wear and when creating your designs.
Another consideration is that we are limited to what stones we can use as beads, and that some beads. For example, a Diamond is well known to be the strongest stone with a Mohs hardness of 10, this also mean that it is very difficult to cut and it unlikely to be cut as a bead. (Herkimer Diamonds are not actual diamonds and technically have a quartz construct). On the other end of the spectrum, traditional Opal is a soft stone, rating a 5.5 on the scale. This is why you will find Opal scratches so easily and needs special care.
Implications of Hardness in Jewelry Making
With this knowledge, it can also have a consideration on your designs. Bracelets and rings have far more wear than necklaces and earrings, which means certain stones may need extra thought before including. Calcite (Mohs 3) and Selenite (Mohs 2) are examples soft stones on the lower end of the hardness scale. If Calcite and selenite are central to your design, you may wish to include spacer beads or knotting to keep it away from other beads or findings that may scratch or damage the softer stones. It's also important to not wet calcite, selenite, or angelite for that matter as it'll deteriorate, but that's a different discussion.
You may also want to consider hardness when deciding which stones you put together in a design. Mohs is tested based on one stone of a higher hardness (say Sapphire or Topaz) being able to scratch a stone of lesser hardness (say Selenite or Calcite). This could mean putting these two together will make your selenite look scratched or broken over time especially if the beads are touching and not buffered by spacers or knots.
When working with softer stones it's also important to give the thread or wire plenty of extra space within the hole to that it can essentially 'float'. If there is too much pressure on the bead from the wire during daily wear it'll eventually crack and break.
Mohs Hardness Scale Tables For Various Stones
Tejas Beads has a wide range of gemstone beads and jewelry making supplies to suit all of your jewelry making needs.